This invention relates to a switch assembly for rotating equipment having a centrifugal operator for moving the switch assembly between open and closed positions, and particularly to a start switch for induction motors.
Rotary apparatus used in industrial arts and other fields often includes speed responsive switches for operating or controlling other devices. An alternately current induction motor is a typical and well known application involving a centrifugally actuated switch assembly. An induction motor including a plurality of windings including a running winding and a phase displaced start winding. The start winding is connected in the circuit during the initial starting of the motor to produce increased starting torque for accelerating the motor from standstill to operating speed. As the motor reaches or approaches operating speed, the starting winding is disconnected from the circuit. A widely used switching system includes a switch assembly mounted within the motor and a centrifugal operator or actuator connected to the rotor shaft and having a spring-loaded element. The spring-loaded element is constructed and arranged to move with a snap-action at a selected speed to actuate the switch assembly for disconnecting the start winding from the circuit.
The switch assembly may be constructed with a fixed contact mounted within the motor and a movable contact mounted on a leaf-spring member. The centrifugal operator is mounted to engage and move the spring member.
A particularly satisfactory structure has the centrifugal actuator located to hold the switch closed against the resilient spring force and movable therefrom to allow the switch unit to open under its spring force. Thus, in a practical installation, the switch unit is held closed by the actuator until such time as the operating or switching speed is established. At that instant the centrifugal actuator responds and moves from the switch assembly with an effective snap action. This releases the spring which then moves to the opened position under or as the result of the spring tension in the spring.
The snap-action movement of the actuator and the switch leaf-spring is important to establish a change between a full-on and a full-off circuit connection. Thus, during the starting period, the switch must be closed to insure energization of the start winding. If the switch is not fully closed, the motor may draw excessive current with a resulting damage to the motor windings. The switch unit is therefore designed to establish a relative large closing force to insure closure of the switch.
The spring member may be specially shaped to produce the desired spring action. In the design, the spring force is preferably selected to permit a high closing pressure without excessive loading of the actuator by the spring member. Excessive loading of the actuator may accelerate wear of the actuator and create erratic actuator functioning. Further, the actuator should move from the spring member with a relative large travel and so rapidly so as to release fully the spring member after which it can rapidly move to the switch open position.
The contact faces may be contaminated with foreign matter which interferes with the closing of the contacts. In addition to providing large closing pressures, various designs have been made to create relative lateral movement between the contact faces so as to remove any foreign matter from the contact faces. For example, U.S. Pat. No. 3,433,908, which issued Mar. 18, 1969 discloses a switch assembly having a movable contact secured to a specially shaped and bent leaf spring which moves with a twisting motion between the open and closed contact positions. The twisting motion creates an action for assisting in the separations of the contacts. Various other designs have provided movement in at least two perpendicular directions to provide improved separating forces as well as a cleaning action during the closing and opening movement of the contacts.
During the opening of the contacts, excessive arcing between the contacts may be created, particularly if the movable contact does not move with sufficient rapidity. Such arcing will tend to erode the contacts and may even create an actual welding of the contacts to each other. Erosion of the contacts will interfere with proper switching action. Welding of the contacts will prevent de-energization of the start winding and probable destruction of the motor. Both contact erosion and possible contact welding can be minimized if not absolutely prevented by proper design of the spring to produce the necessary opening force, as well as movement of the contacts relative to each other to separate contacts that may otherwise become welded.
Thus, in order to ensure a firm contact closure as well as a clean and rapid contact opening, various mechanisms or constructions have been provided for preloading of the contacts and providing for multiple motions of the contact surfaces relative to each other to produce a wiping action between the contact surfaces. The movement of the contacts relative to each other tend to affect a cleaning operation and a break-down of any build-up of material on the contact surfaces, thereby contributing to a more effective switch closure. For example, the previously noted U.S. Pat. No. 3,433,908 discloses a rather complex switch leaf spring unit having an offset spring arm with a deformed spring connection to preload the contact arm and such that the movement of the centrifugal actuator establishes a twisting movement of the contact arm as it moves between the closed and opened position. This is provided to create a corresponding twisting and wiping action between the contacts. Other mechanisms have been employed to impart mutually perpendicular motions to switch contact arm and therefore the contact during the opening and closing movement to effect a desired cleaning action. Generally, such mechanisms involve spring members which are deformed or bent in relatively complex manner mechanical components and spring members. The spring members of the prior art generally require careful and relatively expensive manufacture to adequately and fully establish proper and repeatable switch action. The demands on production techniques are particularly troublesome where mass production of a given design is desired in order to minimize the cost.
However, with a high opening spring pressure, the contact arm tends to bounce or vibrate upon opening. Vibration of the contact arm may result in a corresponding momentary reclosing and opening of the contacts, with possible arcing during opening of the contacts. The vibration of the contact arm may therefore create erratic energization of the start winding, and particularly where the switch assembly is cycled on and off a great number of times during the normal operating life.
Generally, the design of the contact spring member is a compromise of the various spring requirements including sufficient spring pressure and movement to producing an acceptable closure while also preventing erratic movement of the spring mounted contact.
The current design of induction motor also includes a demand for a reduction in the overall size of the switch unit to complement the reduction in the available end space or cavity in which to mount a centrifugal switch assembly. Although the switch mechanism can be physically located adjacent the windings, the magnetic field effect of the winding created by the motor winding may tend to interfere with the optimum switch operation. Thus, the magnetic field associated with the winding may extend the arc within the switch unit, resulting in further damage to the switch unit. Further, the switch unit must of course be connected to the windings and to the power supply. A substantial number of circuit connections is required to the winding on one side and to the incoming power supply to the other side of the mounting.
Further, in many applications the motor is stopped and started many times in any given day or other period. The several components of the centrifugal actuator and the associated switch assembly are therefore subjected to substantial mechanical wear.
As the result of the severe and continuing demands on the design of centrifugal switches, a need for a simple, reliable switch apparatus which can be mounted within a limited cavity, such as the end of an induction motor while maintaining a long-life with reliable switching operation under heavy cycling.